In image forming apparatuses including a laser scanning optical system as an exposure device, a surface-emitting laser (or a vertical-cavity surface-emitting laser) (hereinafter referred to as a VCSEL) has recently been used as a light source of the exposure device to increase the productivity or resolution of the image forming apparatuses. VCSELs have features of easy multibeam formation but, as illustrated in FIG. 12C, a large time delay in a rising edge of a waveform representing an amount of light (hereinafter referred to as a “light-amount waveform”). A delay in a rising edge of the light-amount waveform has an effect on the formation of an electrostatic latent image on a photoconductive drum, and thus leads to a reduction in the quality of an image to be formed on a recording sheet. To address this issue, there has been proposed a method for correcting the rate of rise of the light-amount waveform (see, for example, Japanese Patent Laid-Open No. 63-213983). Japanese Patent Laid-Open No. 63-213983 discloses a method for correcting a rising edge of the light-amount waveform by supplying a correction current to a semiconductor laser in synchronization with the start of the supply of a current based on image data to the semiconductor laser.
There is also known an image forming apparatus configured to execute automatic power control (APC) of a plurality of light-emitting elements over a period during which a non-image area is scanned within a scan period (for example, Japanese Patent Laid-Open No. 2011-207213). Japanese Patent Laid-Open No. 2011-207213 discloses a method for determining a bias current value from a current-light characteristic of a semiconductor laser which indicates the correspondence between a value of a current supplied to the semiconductor laser and an amount of light emitted from the semiconductor laser. In the following, APC for controlling a bias current value is referred to as “bias APC”. The image forming apparatus disclosed in Japanese Patent Laid-Open No. 2011-207213 executes APC of a plurality of light-emitting elements over a period during which a non-image area is scanned within a scan period. The disclosed image forming apparatus performs APC to control a bias current value supplied to the semiconductor laser within a period during which an image-forming area is scanned and which is subsequent to the period during which the non-image area is scanned.
However, the application of a correction current to the semiconductor laser for the control of a bias current value results in the bias current value being set low due to the effect of the correction current.